Various processes by which solid particles can be produced from liquid materials, such as solutions, melts or suspensions, are well known in the art. Of particular interest are the granulation processes, such as that described in Nioh et al. (EP-A-0-026-918). Nioh et al. describe a spouted-bed granulation process, in which a liquid material, in a gas stream, is passed centrally from under and upward through a mass of particles and a number of particles are entrained from this mass by the gas stream and which subsequently, when the velocity of the gas stream decreases, fall back onto the surface of the mass of particles. In this mass of particles there are also present particles originating from a stream of undersized particles and from a stream of oversized particles after being crushed in a size-reducing apparatus.
Another type of granulation process wherein particle growth occurs uses a fluidized bed as the granulator. Such a process is described by Niks et al. in U.S. Pat. No. 4,219,589. In that process, a gas stream atomizes the liquid material to fine droplets which then solidify on nuclei in the fluidized bed. The solidified particles are then removed from the granulator and separated into three streams of particles on the basis of size. The stream of oversized particles are crushed, combined with the stream of undersized particles and returned to the fluidized bed.
Musters in European patent EP-A-0-141-436 describes a fluid bed granulation process in which the liquid material is discharged from a liquid distribution system in the form of a virtually closed, conical film. Nuclei from the bed are moistened with the liquid as they are carried through the conical film with the aid of a powerful gas stream.
The above mentioned granulation processes can be carried out in fluid bed granulators of different shapes. Both box-shaped granulators, as well as cylindrical shaped granulators are known (see e.g. Hans Uhlemann-Lothar Mörl, Wirbelschicht-Spruhgranulation, Springer ISBN 3-540-66985-X, pages 238-241). Box-shaped granulators have a rectangular cross-section in a horizontal plane, whereas cylindrical shaped granulators do have a circular or elliptical cross section in the horizontal plane. Both types of granulators can be characterized in the length (L) and width (W) of the cross section in a horizontal plane. Length L here being defined as the longest horizontal distance between the walls of the granulator, and width as the shortest distance between the granulator walls in the horizontal cross section. The case where L=W in this way characterizes special cases: respectively a square or circular cross section in a horizontal plane. In the field of the present invention it is useful to define oblong granulators as a granulator of any of the above shapes, however with a length that is at least twice as long as its width.
Drawbacks of all of these methods are the significant amount of dust produced during the granulation process or generally present in the granulation unit and the resulting accumulation of the dust in the granulation unit. For the purposes of the present invention, “dust” is defined as particles with a diameter less than 0.5 millimeters. Generally, this dust is carried along by the air stream to the areas of the granulation unit, especially the top, not contacted frequently by granules and deposits there. As the deposits accumulate, large lumps break off and fall down to block the granulator and/or the liquid spray apparatus and thus seriously disturb the granulation process. As a general matter, when this occurs, the granulation process must be stopped and the granulator cleaned. The cleaning procedure and resulting production outage can last 8 to 24 hours depending on such factors as the degree of fouling, composition of the granules and the type of equipment.
The dust generated by and present in a granulation system is caused primarily by three sources.
As a first source of dust there is dust formed by attrition of the granules movements and collisions in the fluidized bed. The amount of dust originating from this source depends very much on the product properties. For many types of product, the relevant properties (hardness, surface structure, abrasion resistance) are such that the amount of dust formed through attrition is rather low.
As a second source of dust, there is dust formed in the process of contacting the liquid introduced into the granulator with the granules in the fluidized bed. The amount of dust formed through this contacting process may vary. In case a spraying concept is used that produces fine droplets (as is the case with many commercial available two-phase sprayers), then such sprayers always produce droplets with a spread in diameter. The finest droplets produced in such sprayers will solidify before hitting a granule and leave the fluidized bed together with the air in the form of dust. If however, the spraying concept used is of the film spraying type, then the amount of dust formed at the sprayers can be very low.
As a third source of dust, there is the dust that is formed in the crushers. The main purpose of the crushing is a limited reduction of the size of the product as it is flowing to the crusher. For instance, if the final product is required in the 2-4 mm range, then typically the task of the crusher will be to produce granules with a diameter in the 1-2 mm range. Inherently to the process of crushing however is the formation of a product fraction with a smaller diameter. Crushed product produced with a diameter less then 0.5 mm shall be classified as ‘dust’, since it will be entrained with the airflow in the granulator.